Temporal and spectral spread submarine locator transmitter, and method and installation using same

ABSTRACT

The invention concerns a temporal and spectral submarine locating method, characterised in that it consists in: equipping the systems or mobile objects to be located and/or identified, with acoustic transmitters transmitting aperiodic signals with a predetermined pseudo-random sequence: deploying, over the working zone where the systems and moving objects to be located are found, one or several measuring points provided, each with an acoustic receiver, with a time scale reference and means for locating said measuring point; transmitting to a processing centre data derived from the measuring points where, at every time of detection on a given frequency band, precise dating is associated in a time scale referential with a position of said measuring point in a spatial referential; the whole set of data derived from the different measuring points being assembled at said processing centre to be operated on by level 2 correlation with the pseudo-random reference sequences corresponding to each of the searched transmitters.

[0001] Purpose

[0002] The invention concerns underwater tracking by acoustic waves, andmore particularly transmitters carried by equipment or mobile units tobe tracked and possibly to be identified. It also concerns a method anda facility to which such transmitters apply.

[0003] Status of the Technology

[0004] Various techniques are used in underwater tracking. In general,they use the propagation of acoustic waves.

[0005] A first method consists of providing the mobile unit to betracked with a transceiver. At a regular rate the mobile unitinterrogates beacons positioned on the bottom of the ocean. Themeasurement of the round trip propagation time of the acoustic waves,with a known velocity of sound in water, makes it possible to calculatethe mobile unit-beacon distances. The coordinates of the beacons beingknown, the mobile unit is then tracked at the intersection of spherescentered on the beacons and radii equal to the propagation times dividedby twice the velocity and corrected, if necessary, by a possible delay.This well-known technique is called tracking by the “long-baselinesystem.”

[0006] Higher performance systems are used by military testing centers.These are torpedo trajectory tracking systems, for example. They arefixed or mobile. In both cases, the points of reception of the acousticsignals are hydrophones located either close to the bottom of the ocean,or suspended from buoys. THOMSON-CSF's European patent No. 0 243 240describes such a device. The mobile units whose trajectory is to betracked are provided with acoustic transmitters. Periodically, every 500ms., for example, they transmit pulse strings chosen from between twofrequencies (a technique called SFSK [Sinusoidal Frequency Shift Keying]in English terminology).

[0007] Other transmitters used in trajectory tracking transmit a firstpulse at a rate of 1 Hz, followed by a pulse offset by a constant factorand of a size proportional to the depth of the mobile unit. Thesetransmitters can have a clock locked on to an external time referenceprior to the launching of the mobile unit. They are then calledsynchronous.

[0008] Also known are acoustic transmitters designed to locate the blackboxes of aircraft sunk in the ocean. They transmit pure 37.5 kHz pulseswhich, although they make it possible to be guided toward the source, donot enable them to be identified. Indeed, the box's transmittercontaining the voice recordings transmits the same signals as the boxcontaining the information relative to the flight's parameters. Thisresults in a complication of the search and recovery operations.

[0009] Existing transmitters have numerous limitations, especially whennumerous mobile units to be tracked by trajectory are presentsimultaneously in the water. This is the case particularly for a groupof underwater divers. In effect, the simultaneous reception, by the samehydrophone, of acoustic pulses from different mobile units posesdetection problems. Included among those problems but not limited heretoare: the poor adjustment of the automatic gain control loop of thepreamplifiers, wave interference phenomena that result from fading or aglut of signals.

[0010] Moreover, not only the interference between the direct wavesshould be taken into consideration, but also interference with multipletrajectories frequently encountered in shallow and medium-depth waters.They are related to the multiple reflections of the signals on thebottom and surface of the ocean.

[0011] Consider, for example, tracking a team of 2, 4 or 8 divers byusing a device involving four buoys located at the surface, stabilizedby anchoring, in a square configuration of 1000 meters per side. Thedivers are fitted with conventional transmitters each of which transmitson its own frequency two acoustic pulses, one synchronous with a timereference (the GPS time, for example), the other making it possible toencode his depth according to the procedure described previously. Forexample, when this pulse is transmitted with a delay of 45 milliseconds,it corresponds to a depth of 30 meters. The following table presents theprobability of detecting without interference the first transmission ofa given diver on at least three buoys, which is the minimum numberrequired to locate the diver without ambiguity using X-Y coordinates.Grid Cell Grid Cell Grid Cell Grid Cell Probability 5 m 50 m 500 m 1000m 2 divers 0% 30% 95% 97% 4 divers 0% 2% 65% 85% 8 divers 0% 0% 30% 60%

[0012] Columns show different grid cell sizes.

[0013] A grid cell of 500 m means that the divers are randomlydistributed in a volume of 500 m×500 m, with a depth of between 20 and40 meters. In the calculation, the multiple trajectory phenomena areassumed to create interference for a duration of 10 millisecondsfollowing the end of the transmission of an acoustic pulse of 5milliseconds duration.

[0014] The table above shows that it is very difficult to locate divers,at a high rate, when they are grouped together inside a small volume anduse synchronous transmitters that have an identical transmissionrecurrence for all of the divers.

[0015] Problem Posed

[0016] In general, the problem posed consists of making it possible totrack and identify a set of underwater objects or mobile units, and doso accurately, without ambiguity, and regardless of the position of themobile units with respect to each other and with respect to thedetection hydrophones. Moreover, the cost of manufacturing thesetransmitters should be low.

[0017] The Invention's Solution to the Problems Posed

[0018] The acoustic tracking transmitters covered by the presentinvention are characterized by the fact that they have electroniccircuits that generate aperiodic pulsed acoustic signals, thetransmission dates of which are distributed according to a known,predetermined pseudo-random sequence, both at the transmission equipmentitself, as well as at the receiving equipment or processing center.

[0019] According to another characteristic, said electronic circuits areconfigured so that the aperiodic pseudo-random sequence of pulses theygenerate is a sequence of signals encoded by hops in wide or narrowfrequency bands, according to an order that is predetermined and known,both at the transmission equipment itself, as well as at the receivingequipment or processing center.

[0020] The facility according to the invention is notable by the factthat it comprises:

[0021] transmitters suitable for generating aperiodic pulsed acousticsignals, integrated into equipment or mobile units to be tracked and/oridentified;

[0022] one or more points of measurement provided with an acousticreceiver, a time scale reference and a means of tracking said point ofmeasurement;

[0023] and one or more processing centers that make it possible toidentify the transmitters by level 2 correction [sic] calculationsbetween a series of moments of reception of pulses and the pseudo-randomsequences peculiar to each transmitter.

[0024] Alternatively, the point(s) of measurement can be fixed andpreviously tracked by any means of positioning in a local or geographicreferential system.

[0025] The method of the invention is notable in that the equipment ormobile units to be tracked and/or identified are equipped with acoustictransmitters transmitting aperiodic pulsed acoustic signals according toa predetermined pseudo-random sequence; in the work area where theequipment or mobile units to be tracked are located, one or more pointsof measurement are deployed, each of which points is provided with anacoustic receiver, a time scale reference and a means of tracking saidpoint of measurement; the information produced by the points ofmeasurement is transmitted to a processing center where, at each momentof detection on a given frequency band, it is associated with a precisedating in a time scale referential system and a position of said pointof measurement in a spatial referential system; all of this informationfrom different points of measurement is collected at said processingcenter so that it can be processed by level 2 correlation with thepseudo-random reference sequences pertaining to each of the transmittersbeing sought.

[0026] Alternatively, the level 2 correlation processing can be done atthe point(s) of measurement.

[0027] As will be seen further on, in the description of one form ofembodiment, one particular solution to the problem posed consists ofhaving each mobile unit transmit cyclically a series of pulsed andaperiodic acoustic signals.

[0028] The invention will be better understood and other characteristicswill become apparent from the following description, with reference tothe attached figures.

[0029]FIG. 1 represents the time sequencing of the transmitted pulses.

[0030]FIG. 2 represents a flow chart of a transmitter of pulsed andaperiodic acoustic signals.

[0031]FIG. 3 represents the spatial spread of the transmitted pulses.

[0032]FIG. 4 represents the temporal occupancy of the codes transmittedusing conventional trajectory tracking transmitters used in synchronousmode.

[0033]FIG. 5 represents the temporal occupancy of pulses transmittedusing the temporal spread method that is an object of the presentinvention.

[0034]FIG. 6 represents the decoding key.

[0035] The table provided above shows the probabilities of detection ofan acoustic transmission produced by a transmitter at at least threepoints of reception, with no interference, using conventional trajectorytracking transmitters used in synchronous mode.

[0036] In general, the means of tracking underwater mobile units, usingthe transmitters covered by the present invention, is comprised of a setof widely spaced receiving points. In the following discussion, we willassume that these receiving points are comprised of hydrophones[suspended] vertically from buoys. The points of reception haveelectronics for detection and tracking of said point of reception.

[0037]FIG. 1 shows an example of pseudo-random sequencing of temporalspread pulses P0 to P4. In this particular case, 5 pulses aretransmitted cyclically. They are characterized by the time intervals T1,T2 to T5 separating each transmission.

[0038] In FIG. 2 there is an energy source (1), such as a battery, aclock circuit (2), which could be quartz, a memory (3) in which arestored or calculated the time intervals, t_(i) separating eachtransmission and a logic circuit for sequencing said transmissions. Thecircuit (4) represents the power transmission stage that enables a highvoltage to be generated to activate the transmission transducer (5), ingeneral a piezo-electric ceramic. A pressure sensor (6) can beintegrated into the unit. It is then used for depth telemetry byinterleaving into the first sequence of pulse transmissions a secondsequence using another pseudo-random sequencing, offset from the firstby a time proportional to the depth of the mobile unit. An externalsynchronization circuit (7) makes it possible, if necessary, tosynchronize the clock (2) to a common time scale reference shared by thetransmitter and the receivers. An optional output (8) of the timereference can be used for marking measurements used by another devicenot represented.

[0039]FIG. 3 represents in time/frequency space the combination of thetemporal spread of the pulsed codes T1 to T5 and pseudo-random hops inthe spacing of frequencies F1 to F3. The result is a frequency as wellas temporal spread of the transmitted codes, producing a very largediversity in the choice of codings. They are characterized by amultidimensional (time and frequency) key.

[0040]FIG. 4 represents what the temporal occupancy would be of signalstransmitted by a group of four underwater divers if they were workinginside a square 500 meters on a side, each of them having a conventionaltransmitter sharing a common time reference and having a depth telemetrydevice. The concentration of the signals within the 0.3 to 0.7 secondinterval is the cause of the collisions between the signals and explainsthe small probability of detection without interference on at leastthree points of measurement. The horizontal axis of the figurerepresents, in a cycle of one second, the moments of arrival of thesignals transmitted by the divers at four buoys located at the fourcorners of a square 1000 meters on a side. The vertical axis representsa succession of 10 transmission cycles offset by one second.

[0041]FIG. 5, representative of the invention, shows what the temporaloccupancy would be of pulsed signals transmitted by a group of fourunderwater divers if they were working within a square 500 meters on aside, each of them having a transmitter according to the invention,furnished with the device for the temporal spread of the transmittedcodes. The result is a distribution of codes transmitted over the entire0-1 second space. There is therefore only a slight probability ofcollision of the signals at the receivers. The horizontal and verticalaxes of this figure have the same meaning as those in FIG. 4.

[0042]FIG. 6 presents the decoding key corresponding to a sequence ofpulsed codes transmitted by a transmitter using only one frequency band.The width of each element of the key is determined in order to take intoaccount the fluctuations of the arrival moments related to thevariations in speed of the transmitter during the period of time inquestion. The detection is accomplished by correlation between the keyand the signal detection peaks in the frequency band in question. Thevalue of the correlation function takes discrete values varying from 0to a maximum of “n,” “n” being the maximum number of transmitted codesthat can be detected inside the observation window. This correlationfunction is called level 2, in contrast with level 1 correlation thatcould have been used to detect each individual pulse.

[0043]FIG. 3 presents a multidimensional key F1, F2, F3 corresponding toa coding that is also spread over three distinct frequency bands.

[0044] The use of acoustic transmitters transmitting aperiodic pulsedsignals in a predetermined pseudo-random sequence, an object of thepresent invention, has the following advantages:

[0045] Because of the technologies of electronic components such asPICs, the manufacture of transmitters having the ability to transmitaperiodic pulsed signals is inexpensive.

[0046] By fixing a different pseudo-random sequence in a non-volatilememory of each transmitter, it is then very easy at the receiver toidentify the transmitter as well as to date the moment of reception of aseries of codes with respect to a timing reference; moreover, this isdone without ambiguity of cycle number or origin of the source. This isabsolutely not the case today with the aircraft black box transmittersthat transmit on the same frequency and at the same time interval. Thisunique characteristic provides the valuable possibility ofdifferentiating between transmitters, and thus the item to which it isattached. This can prove useful in distinguishing items that are lost onthe bottom of the ocean (containers of dangerous chemicals, nuclearmaterials, weapons, etc.) and thus enable them to be recovered inaccordance with different priorities.

[0047] The probability that a peak of the level 2 correlation will notcorrespond to the selected mobile unit is extremely low. There istherefore an excellent probability of simultaneous detection at severalpoints of reception of the pseudo-random sequences of impulsescorresponding to a large number of mobile units present in a given area,even if said area is small in size.

[0048] Because of the very large number of different pseudo-randomsequences that can be generated, the detection of noise instead of areal signal, as well as the non-detection of a real signal, has littleinfluence on the results of the calculation of the level 2 correlation.The result is a very high insensitivity to ambient noise, decoys, aswell as deliberate jamming.

[0049] Moreover, by acting only on the aperiodic aspect of thetransmitters, it becomes possible to improve search equipment whilepreserving compatibility with existing relocation equipment that isbased on purely directional measurements.

[0050] Implementation of the Tracking and Identification Method

[0051] Equipment to be tracked and/or identified is equipped withacoustic transmitters, transmitting aperiodic pulsed signals accordingto a predetermined pseudo-random sequence.

[0052] In the work area where the equipment to be tracked is located,one or more points of measurement are deployed, furnished with anacoustic receiver, a time scale reference and a means of tracking saidpoint of measurement. The points of measurement and/or the processingcenter(s) are arranged on the surface, or immersed in the water, or atthe bottom of the ocean, fixed, or drifting, or movable.

[0053] The information from the points of measurement is transmitted toa processing center where, at each moment of detection on a channel ofthe arrival of a pulse (represented by a given frequency band), aprecise dating is associated within a time scale reference system and aposition of said point of measurement within a space reference system.

[0054] All of this information from different points of measurement iscollected at said processing center in order to process it by level 2correlation with the reference pseudo-random sequences corresponding toeach of the transmitters sought.

[0055] The sole existence of a maximum of the level 2 correlationfunction, as previously defined, above a given threshold, makes itpossible both to be certain of the presence of transmitter in the area,as well as to identify it.

[0056] A propagation time of the signals transmitted toward the receivercorresponds to the peak of each level 2 correlation function.

[0057] Based on all of the propagation times of the signals correlatedto different receivers corresponding to a same cycle of transmissions,it is then possible to calculate the position of the mobile unit byusing well-known tracking algorithms called “circular” or “hyperbolic,”depending on whether or not the time scale reference of the transmitteris synchronous with the time scale reference of the receivers.

[0058] Thus, the position of the transmitters can be determined withprecision, without ambiguity.

1. Acoustic tracking transmitters, characterized in that they haveelectronic circuits that generate aperiodic acoustic pulsed signals, thetransmission dates of which are distributed according to a known,predetermined pseudo-random sequence, both at the transmission equipmentitself, as well as at the receiving equipment or processing center. 2.Acoustic tracking transmitters according to claim 1, characterized inthat they have electronic circuits so that the aperiodic pseudo-randomsequence of pulsed signals they generate is a sequence of pulsed signalsencoded by hops in wide or narrow frequency bands, according to an orderthat is predetermined and known both at the transmission equipmentitself, as well as at the receiving equipment or processing center. 3.Acoustic tracking transmitters according to either of claims 1 or 2,characterized in that they have electronic circuits that enable one ormore sequences of interleaved sequences of pulses to be transmitted forpurposes of the remote transmission of the depth of the mobile unit orany other information.
 4. Underwater tracking facility comprisingacoustic tracking transmitters according to any of claims 1 to 3,integrated into equipment or mobile units to be tracked and/oridentified, characterized in that it has one or more points ofmeasurement provided with an acoustic receiver, a time scale referenceand a means of tracking said point of measurement, and one or moreprocessing centers that make it possible to identify the transmitters bylevel 2 correlation calculations between a series of moments ofreception of pulsed signals and the pseudo-random sequences peculiar toeach transmitter.
 5. Tracking facility according to claim 4,characterized in that the points of measurement are situated on thesurface, immersed in the water or placed at the bottom of the ocean,fixed, drifting, or movable.
 6. Tracking facility according to claim 4,characterized in that the points of measurement are fixed and of aposition determined by any means of tracking in a local or geographicreferential system.
 7. Underwater tracking method by temporal andspectral spread, characterized in that the equipment or mobile units tobe tracked and/or identified are equipped with acoustic transmitterstransmitting aperiodic pulsed acoustic signals according to apredetermined pseudo-random sequence; in the work area where theequipment or mobile units to be tracked are located, one or more pointsof measurement are deployed, each of which points is provided with anacoustic receiver, a time scale reference and a means of tracking saidpoint of measurement; the information produced by the points ofmeasurement is transmitted to a processing center where, at each momentof detection on a given frequency band, it is associated with a precisedating in a time scale referential system and a position of said pointof measurement in a spatial referential system; all of this informationfrom different points of measurement is collected at said processingcenter so that it can be processed by level 2 correlation with thereference pseudo-random sequences of pulses pertaining to each of thetransmitters being sought.
 8. Underwater tracking method according toclaim 7, characterized in that the points of measurement and/or theprocessing center(s) are arranged on the surface, immersed in the water,or at the bottom of the ocean, fixed, drifting, or movable. 9.Underwater tracking method according to claim 7, characterized in thatthe level 2 correlation is performed at the point of measurement or atthe processing center.